1. Field of the Invention
The present invention relates to a recording medium suitable for recording with an ink, a process for production of the recording medium, and an image-forming method employing the recording medium. Particularly, the present invention relates to an ink-jet recording medium which is capable of forming images with high image density and clear color tone, and has high ink absorbency, to a process for production of the recording medium, and to an image-forming method employing the recording medium.
2. Related Background Art
The ink-jet recording is a method for recording images and letters by ejecting fine droplets of ink onto a recording medium such as a paper sheet. The ink-jet recording is becoming popular rapidly in recent years in various applications because of its high recording speed, low noise generation, ease of multicolor recording, flexibility in pattern recording, and needlessness of image development and fixation. Multicolor ink-jet recording is coming to be used in full color image recording since it is capable of giving images comparable with images formed by multicolor gravure printing or color photography, and is less expensive than multicolor printing when the number of reproduction is small.
With improvements of the ink-jet recording apparatus and method in recording speed, fineness of recording, and full color recording, the recording medium is also required to have higher qualities. Hitherto, various types of recording media have been disclosed to meet the requirements. For example, JP-A-55-5830 ("JP-A" herein means Japanese Patent Laid-Open Publication) discloses an ink jet recording paper sheet which has an ink-absorbing coating layer provided on a supporting paper sheet; and JP-A-55-51583 discloses use of noncrystal silica as a pigment in a coating layer. U.S. Pat. No. 4,879,166, U.S. Pat. No. 5,104,730, JP-A-2-276670, JP-A-5-32413, and JP-A-5-32414 disclose recording sheets having an ink-receiving layer containing alumina hydrate of pseudo-boehmite structure.
Conventional recording media, however, have disadvantages as follows.
One disadvantage is that conventional recording media are insufficient in adsorption and coloring state of a dye contained in the ink, and do not give high optical density of printed areas. To offset the disadvantage, JP-A-5-32414 discloses a recording medium employing alumina sol having an interplanar spacing of (020) plane of not more than 6.17 .ANG. (0.617 nm), and describes that a smaller interplanar spacing provides high optical density of printed areas. However, a smaller interplanar spacing of the alumina hydrate makes the surface thereof hydrophobic, which causes another disadvantage of low absorbency for the solvent component of the ink to result in low image quality owing to ink repulsion in the printed area, or to result, with a dye of high hydrophilicity, in low optical density, or bleeding or beading of ink dots. A smaller interplanar spacing brings also a disadvantage of low bonding strength of the alumina hydrate with a binder which is a hydrophilic resin to cause powder-falling or cracking of the ink-receiving layer.
Another disadvantage of conventional recording media is that the ink-receiving layer formed by using a porous material is not sufficiently transparent, causing white-turbidity or insufficient optical density of the printed area. To offset the disadvantage, JP-A-5-32413 and JP-A-5-32414 disclose a transparent low-haze alumina sol having a crystallite size of not less than 60 .ANG. (6.0 nm), or not less than 70 .ANG. (7.0 nm) in a direction perpendicular to (010) plane, and disclose also a recording medium employing the alumina sol. On the other hand, JP-A-59-3020 and a report in "Keikinzoku" (Light Metal), Vol. 22, No. 4, pp. 295-308 show that the crystal structure of the alumina hydrate is changed by a heat treatment or a dispersion treatment. A report in "Clays and Clay Minerals", Vol. 28, No. 5, pp. 373-380 (1980) discloses that the crystal structure of the alumina hydrate is changed by drying conditions of the dispersion. Therefore, even when the alumina hydrate having a controlled interplanar spacing of (020) plane and a controlled crystallite size in a direction perpendicular to (010) plane is used for preparation of a recording medium, the interplanar spacing and the crystallite size are not always the same as those of the starting alumina hydrate or alumina sol in the formed recording medium produced through coating and drying steps after a coating dispersion has been prepared by adding a binder to the alumina hydrate. Therefore, the above-cited documents do not describe the method for obtaining a recording medium prepared through a series of steps from a dispersion of alumina hydrate, which has a controlled interplanar spacing of the alumina hydrate of (020) plane and the controlled crystallite size in a direction perpendicular to the (010) plane in the resulting recording medium.
A still another disadvantage of conventional recording media is that they have an insufficient transparency of the ink-receiving layer in an application to observe an image with transmitted light such as overhead projector (OHP) films and in obtaining a high optical density. U.S. Pat. No. 5,104,730 and JP-A-2-276670 disclose a recording medium having a porous ink-receiving layer which has a porous layer having a volume of pores with a pore radius exceeding 100 .ANG. (10.0 nm) at not larger than 0.1 cc/g (cm.sup.3 /g), and having a low haze. However, the transparency of the ink-receiving layer is not improved by merely controlling the pore diameter and the pore volume, since the transparency is greatly affected by a crystallite size.
A further disadvantage of conventional recording media is that, in color image printing in which inks are applied in a larger amount onto a recording medium, the ink flows out, or the recorded image spreads to impair the image quality, or printed image density becomes low. To offset this disadvantage, JP-A-58-110288 and JP-A-2-267760 disclose a recording medium having a pore size distribution controlled to have peaks at a specified pore radius. This is based on the idea that ink absorbency, printed image density, and image resolution depend on the pore diameter distribution and the pore volume. This method, however, does not provide a sufficiently high density and a resolution of the image. This problem is not solved by merely controlling the pore diameter distribution and the pore volume without controlling the crystal structure.